New TAXI-type xylanase inhibitor genes are inducible by pathogens and wounding in hexaploid wheat.

TAXI-I (Triticum aestivum xylanase inhibitor I) is a wheat grain protein that inhibits arabinoxylan fragmentation by microbial endo-beta-1,4-xylanases used in the food industry. Although TAXI was speculated to be involved in counterattack against pathogens, there is actually no evidence to support this hypothesis. We have now demonstrated the presence of TAXI family members with isolation of two mRNA species, Taxi-III and Taxi-IV. At the nucleotide sequence level, Taxi-III and Taxi-IV were 91.7% and 92.0% identical, respectively, to Taxi-I, and Taxi-III and Taxi-IV were 96.8% identical. Accumulation of Taxi-III/IV transcripts was most evident in roots and older leaves where transcripts of Taxi-I were negligible. When challenged by fungal pathogens Fusarium graminearum and Erysiphe graminis, the concentrations of Taxi-III/IV transcripts increased significantly. In contrast, the increases in Taxi-I transcripts in response to these pathogens were rather limited. Both Taxi-I and Taxi-III/IV were strongly expressed in wounded leaves. The upstream region of Taxi-III contained W boxes and GCC boxes, which are sufficient to confer pathogen and wound inducibility on promoters. Recombinant TAXI-III protein inhibited Aspergillus niger and Trichoderma sp. xylanases: it was also active against some spelt xylan-induced xylanases of F. graminearum. These features suggest that some, but not all, TAXI-type xylanase inhibitors have a role in plant defense.